The present invention relates, in general, to voltage converters and, more particularly, to buck voltage converters, which can be externally programmed for switched or linear modes of operation.
Direct Current (DC) power converters typically employ either linear conversion or switched conversion techniques to convert one DC voltage to a second DC voltage. DC power converters are necessary in most mobile electronic devices from mobile cellular telephone handsets to portable compact disc players.
Both switched mode and linear mode voltage converters employ a power Metal Oxide Semiconductor Field Effect Transistor (MOSFET) to conduct a current in response to a drive signal delivered by the voltage converter. The current conducted by the MOSFET is used to charge a storage device, such as a capacitor, which provides a source of operating potential to the voltage converter load. In a switched mode of operation, an internal clock signal sets the drive signal to the MOSFET. Once the voltage across the capacitor has been set to a predetermined voltage level, a feedback signal triggers a logic circuit to cancel the drive signal to the MOSFET and thereby prevent further charging of the capacitor. The drive signal developed by the switched mode converter typically operates between a minimum and a maximum duty cycle directly proportional to loading conditions. In other words, under high loading conditions, the duty cycle is at a maximum and under low loading conditions, the duty cycle is at a minimum.
Linear mode voltage conversion devices do not employ a switching drive signal, instead the drive signal is constantly applied to the gate of the power MOSFET. The drive signal is used to set the on resistance of the p-channel MOSFET when the device is within the low-dropout region. When the voltage across the gate terminal and the source terminal of the MOSFET is lower than the threshold voltage of the MOSFET, the on resistance of the MOSFET is set low. Conversely, when the voltage across the gate terminal and the source terminal of the MOSFET is higher than the threshold voltage, the on resistance of the MOSFET is set high. A low on resistance produces an increased voltage level at the output terminal of the linear mode voltage converter. A high on resistance produces a decreased voltage level at the output terminal of the linear mode voltage converter. In a linear mode of operation, a feedback signal is used to set the magnitude of the gate drive voltage and thus provide the required regulation.
Switched mode converters can provide high efficiency with noisy output at normal to high loading conditions. Linear mode converters can provide a low-noise, fast response output with poor efficiency. Prior art applications of buck voltage converters exist as either switched mode or linear mode conversion devices. Prior art switched converter applications, therefore, exhibit good voltage conversion efficiency under normal to high loads, but suffer at low load when the switched converter has transitioned to its minimum duty cycle. Prior art linear mode converter applications provide low noise voltage conversion, but the efficiency suffers.
Accordingly, it would be advantageous for a buck voltage converter to provide both switched mode and linear mode conversion operation to obtain optimal selectivity among noise, efficiency and response time under varying loading conditions. Additionally, it would be advantageous to allow for external control of the conversion mode by, for example, a micro-controller, so that the power conversion mode can be selected over a multitude of loading scenarios.